The present invention relates to a K-edge filter for X-ray, which is used for obtaining predetermined spectra in X-ray apparatuses such as a medical X-ray diagnostic apparatus, a bone density measuring apparatus, a non-destructive inspection apparatus, an X-ray analyzer or the like and an X-ray apparatus employing the K-edge filter.
Generally, X-rays generated by an X-ray generator are constituted by photons having various energy levels and have an energy spectrum in which a characteristic X-ray spectrum of a steep waveform is added to gentle continuous X-ray spectrum as shown in FIG. 12. Absorption of X-rays transmitted through a substance is caused either by a phenomenon (1) in which X-rays produce a photoelectric effect in the substance so as to emit photoelectrons such that photons vanish or by a phenomenon (2) in which X-rays are partially scattered during travel of the X-rays through the substance. The absorption of X-rays caused by the phenomenon (2) exhibits a noncontinuous change (referred to as an "absorption edge") in the attenuation coefficient (absorption coefficient). The absorption edge based on K-shell electrons is referred to as a "K-absorption edge" and noncontinuous change characteristics of the attenuation coefficient are employed for an X-ray filter.
In an X-ray diagnostic apparatus, a non-destructive inspection apparatus, an X-ray analyzer or the like, it is a common practice that the X-rays used are measured by limiting the wavelength range or X-rays having a plurality of limited wavelength ranges are measured so as to be compared with one another. Meanwhile, in an apparatus for measuring a substance, for example, a bone mineral densitometry, by dividing the wavelength of the X-rays into a plurality of wavelength ranges, namely, by employing measurement results of a plurality of X-rays made monochromatic in a pseudo manner, a calculation is performed so as to effect a measurement. A K-edge filter is used as an X-ray filter for separating energy spectrum of X-rays into high and low energy regions. The K-edge filter is made of a material which not only has a K-absorption edge in a target energy region of X-rays but possesses a dependence of H's attenuation coefficient upon energy as shown in FIG. 13. The amount of X-rays which have passed through the K-edge filter changes markedly before and after the K-absorption edge, so that energy spectrum of the X-rays is separated into the two energy regions.
FIG. 14 shows an X-ray spectrum obtained after X-rays have passed through a K-edge filter made of gadolinium (Gd) and having a thickness of 100 .mu.m. It will be seen from FIG. 14 that the energy spectrum of X-rays is separated into two energy regions at a K-absorption edge of Gd of 50.2 keV. Such conventional K-edge filters are usually made of only one element having a K-absorption edge in the X-ray region, for example, cerium (Ce), samarium (Sm) or the like.
Meanwhile, in a measuring apparatus employing a K-edge filter, an X-ray detector is usually formed by the combination of a scintillator of NaI or GdWO.sub.3 and a photomultiplier tube. A K-edge filter used for the X-ray detector is made of Sm or Ce or the like.
However, the prior art arrangements referred to above have the following drawbacks. In the known K-edge filter made of a single element as shown in FIG. 14, the difference between an effective energy of the low energy region and that of the high energy region of the separated energy spectrum of the X-rays is small and the amount of X-rays becomes large in the vicinity of the boundary of the energy separation. Therefore, the energy spectrum of X-rays cannot be distinctly separated into high and low energy regions. Furthermore, since the width of the spectra of the two energy regions becomes large, a plurality of monochromatic X-rays cannot be produced in a pseudo manner. Meanwhile, if the thickness of the K-edge filter made of a single element is increased so as to clearly separate the energy spectrum of the X-ray into high and low energy regions, the number of X-ray photons passing through the K-edge filter decreases undesirably.
In an X-ray detector, the characteristic X-ray proper to the substance forming the X-ray detector is produced by an incident X-ray. When this characteristic X-ray is again absorbed into the X-ray detector, an output pulse signal accurately representing the energy of an incident X-ray can be obtained. However, if the characteristic X-ray is emitted out of the X-ray detector without being absorbed thereinto, namely, if the characteristic X-ray escapes, only a pulse signal having a pulse height smaller than that corresponding to the energy of an incident X-ray is outputted. In other words, X-rays are detected at this time as if X-rays having an energy smaller than that of the actual incident X-rays are incident upon the X-ray detector. This phenomenon is generally referred to as "characteristic X-ray escape". Output pulses having a pulse height lowered by this phenomenon are referred to as "escape peak of K-shell characteristic X-rays". The frequency of occurrence of characteristic x-ray escape depends on the volume of the X-ray detector and becomes larger the volume of the X-ray detector is reduced.
When ordinary X-rays having an energy spectrum shown in FIG. 12 is detected, the pulse height distribution of output pulses is obtained as shown by the curve a in FIG. 15. This pulse height distribution contains a pulse height component due to characteristic X-ray escape as shown by the curve b in FIG. 15. Meanwhile, also in the case where the energy spectrum of the X-rays is separated into high and low energy regions by passing X-rays through the K-edge filter and is detected, the pulse component due to the characteristic X-ray escape exists. Therefore, in the case where the energy spectrum of X-rays is separated by the K-edge filter into high and low energy regions by employing energy in the vicinity of the K-absorption edge as the boundary of the high and low energy regions so as to be measured such that the data on the numbers of photons present in the high and low energy regions are utilized, even photons having an energy of the high energy region appear as signals partially in the low energy region owing to the characteristic X-ray escape. As a result, it is impossible to obtain the accurate energy distribution of photons incident upon the X-ray detector.
For example, in an NaI scintillation detector, characteristic X-rays of about 1 keV and 28.3-33.2 keV are, respectively, produced for Na and I. In the above characteristic X-rays, especially the characteristic X-rays for I poses a problem. FIG. 16 shows results of measurement in which X-ray emitting photons of a maximum energy of 80 keV are measured by the NaI scintillation detector through its energy separation based on a K-edge filter of Ce having the K-absorption edge at 40.4 keV. The NaI scintillation detector is operated in a photon counting mode and the pulse height of output pulses of the NaI scintillation detector is proportional to the energy of incident X-ray photons. In the abscissa of FIG. 16, pulse height is converted into the energy of photons. When characteristic X-rays of I have escaped, only pulses having a pulse height corresponding to an energy 28.3-33.2 keV lower than energy of incident photons are outputted. For example, assuming that X-ray photons of 70 keV are incident upon the NaI scintillation detector and characteristic X-rays of I escape, pulses having a pulse height corresponding to an energy of 36.8-41.7 keV are outputted. An effective energy of an output peak at the side lower than a separation energy of 40.4 keV is 38 keV, while an effective energy of an output peak at the side higher than the separation energy is 74 keV. In FIG. 16, the curve b illustrates the output due to characteristic X-ray escape. As shown by the hatching in FIG. 16, an effective energy of the X-ray escape peak induced by the incident X-ray at the side higher than the separation energy is 44 keV.
When total counts of signals corresponding to an energy not less than the separation energy of 40.4 keV and the total counts of signals corresponding to energy not more than the separation energy of 40.4 keV are obtained as data, the signals shown by the hatching in FIG. 16 are produced by the incidence of photons in the high energy region separated by the K-edge filter but are measured at the sides which are both higher and lower than the separation energy based on the K-edge filter. In this example, 40% of the signals shown by the hatching in FIG. 16 are measured at the side higher than the separation energy.
By using the NaI scintillation detector, X-ray photons having a maximum energy of 100 keV are measured through the energy separation based on the K-edge filter in the same manner as described above. FIG. 17 shows results of pulse height analysis in the case of the K-edge filter made of Sm. Sm has a separation energy of 47 keV. An effective energy of the peak at the side lower than the separation energy is 45 keV, while an effective energy of the peak at the side higher than the separation energy is 80 keV. As shown by the hatching in FIG. 17, an effective energy of the escape peak of the characteristic X-rays induced by the incident X-rays at the side higher than the separation energy is 50 keV. Since the separation energy of 47 keV is smaller than the effective energy of the 50 keV of the escape peak of characteristic X-rays induced by the incident X-rays at the side higher than the separation energy, about 40% of the output pulses based on the characteristic X-ray escape appear at the side higher than the separation energy.
In the case where signals based on the characteristic X-ray escape are measured at the sides which are both higher and lower than the separation energy, this influence should be corrected. However, as the number of such signals is increased further, it becomes more difficult to correct for the influence, so that the effects of correction of the influence diminish and the measurement accuracy of the substance, etc. deteriorates.